500 T ho day. — On the Organization of Growth and 
formed only when growth in length comes to an end. But even when 
these are pitted vessels they are smaller in diameter than the spiral vessels 
formed during the period of most active growth of the bundle, and far 
smaller than the pitted vessels typical of the secondary wood (see Text-figs. 8 
and 9). The bundle may therefore be said to have a grand period of 
transverse growth which corresponds to that of the leaf to which it belongs 
but is more or less independent of that of the internode in which it 
occurs. 
At the end of this period the cambium becomes sluggish and remains 
so for a considerable time. In the large bundles of the upper part of the 
stem the cambial zone often develops air-spaces and becomes parenchy- 
matous, especially the median part, so that a definitive cambium cannot be 
located. This applies throughout the upper part of the bundle where its 
development is completed during the primary phase. 
0 . 0-5 1 - 0 mm. 
2 3 
Text- fig. 8. Sections of the median trace bundle of the leaf (already mature) at the seventh 
node of a vigorous plant : (i) in internode VII; (2) in internode V ; (3) in internode IV- In the 
last, the small amount of primary xylem is supplemented by secondary xylem (cross-hatched) with 
large pitted vessels. 
Farther down, where the primary growth of the parenchyma comes to 
an end while the leaf to which the bundle belongs is still expanding, the 
further growth of the bundle is affected and the additional xylem, although 
it is formed by the same cambium, develops the histological characters of 
secondary wood, with much larger pitted vessels, solid bands of fibres, and 
secondary medullary rays. 
This formation of secondary xylem, once it has well begun, continues 
steadily and the cambium does not become dormant as in the internodes 
above. This is illustrated by Text-fig. 9, which represents bundles from 
one and the same section of an old stem. They are the median bundles 
belonging to leaves of successive nodes above the plane of section, and 
therefore of diminishing age ; but the youngest, D, has formed far more 
xylem than the others. Here, too, it includes large pitted vessels which 
have been produced in normal proportion along with the wood fibres 
throughout the period of growth. In bundle C the secondary xylem is 
